Joining of structural components made from polymer composite materials, particularly in the aerospace industry, generally involves the use of mechanical fasteners. There are several reasons for this. The components that are joined to make aerospace assemblies often have a complex geometry. Components may be singly or doubly curved, and have corners with tight radii. Additionally, there is a requirement for maintaining tight tolerances in component location and final assembly shape. The process for incorporating mechanical fasteners into a composite assembly is expensive, generally requiring several hours of time on expensive automated machines. In addition, composite materials often do not perform optimally with mechanical fasteners—in most cases assemblies are more efficient when joined with an adhesive.
Adhesives have also been used widely for joining composites, but less widely in the aerospace industry. Most structural adhesive bonding processes require a rigorous surface preparation stage, adding substantial expense to the process. Furthermore, the adhesives generally require elevated and carefully-controlled temperature and pressure in the cure cycle, which mostly involves placing an entire assembly inside an oven or autoclave. In order to obtain tight tolerances expensive tooling is required. This tooling also requires cleaning and surface preparation for each assembly.
An alternative method for joining composite components is to weld components that have a thermoplastic surface. Some advantages in using this process are the potential time, cost and weight savings compared to the use of mechanical fasteners, as well as the potential for easy dismantling of assemblies. However, to provide a widely applicable means of low-cost assembly, this technology also has to allow easy and accurate means of location. The ease with which this can be done is closely related to the required welding pressure.
A significant part of the prior art concerning the welding of polymer composite components with thermoplastic surfaces relates to the welding of thermoplastic composite components, where the surface is the same material type as the polymer matrix in the composite. While this process is conceptually simple, in practice it has proven to be unattractive, with high compaction pressures (usually 500 kPa or higher) required during welding to maintain the integrity of the components due to conduction of the welded zone heat away from the surface. Subsequent research has turned to the application of thermoplastic surfaces to thermosetting composites. An example of this process is described in U.S. Pat. No. 5,304,269. This patent describes the partial embedding of thermoplastic and thermosetting resins into a layer of dry fibre reinforcement, along with compatible amorphous and semi-crystalline thermoplastics to make a weld. The process described in U.S. Pat. No. 5,643,390 includes establishing a semi-interpenetrating polymer network between an amorphous thermoplastic and the thermosetting polymer. Both of these techniques could be used to join thermosetting polymer composite components, and have the advantage that, in principle, lower welding pressures could be used to join components. However, in order to provide adhesive strength and high service temperature, high melting temperature semi-crystalline thermoplastics (in the case of U.S. Pat. No. 5,304,269) and amorphous thermoplastics (in the case of U.S. Pat. No. 5,643,390) were used, where these in general had a softening temperature in excess of the degradation temperature of the thermosetting composite substrate.
In application both high temperatures and high pressures were required to create a quality weld. Unfortunately the use of high pressures in turn requires the use of substantial tooling to provide high-accuracy assemblies, making assembly more expensive, and in the above cases lessening the appeal of using of thermoplastic-surfaced thermosetting composites. This is particularly relevant to the assembly of aircraft components, which often have curvature or twist, and therefore require a means of assembly that accommodates these geometries.
The applicant has recently developed a process involving the integration of a semi-crystalline thermoplastic onto the surface of a thermosetting composite through the formation of a semi-interpenetrating polymer network. This process is described in applicant's international patent publication No. WO 03/011573, the contents of which are incorporated herein by reference. This process has several advantages including chemical resistance in service and ease of assembly, disassembly and reassembly. However, key advantages include the low welding pressures required to make an effective joint, with as little as 100 kPa required during the formation of a high-quality weld, and the ability to conduct welding outside a clean-room environment. It is now possible, therefore, to consider the low-cost assembly of thermoplastic-surfaced thermosetting composite components.